Living in a liquefaction zone — Bay Area's riskiest neighborhoods

If the Bay Area is indeed overdue for the "Big One," as many scientists believe, it's important to know what kind of soil your home is built on.

"The most recent scientific study indicates that for the entire San Francisco Bay Area there is about a 70% chance of a magnitude 6.7 or greater in the next 30 years occurring on one of the several earthquake faults that run through the area," geophysicist Tom Brocher with the U.S. Geological Survey said last year.

Photo: USGS

In this USGS liquefaction susceptibility map of San Francisco, areas of very high risk are marked in red, high risk in orange, moderate risk in yellow, low risk in green and very low risk are white.

In this USGS liquefaction susceptibility map of San Francisco,...

Brocher said the most likely suspect for the next big quake is the Hayward Fault Zone, a 74-mile-long fault line in the East Bay that hasn't produced a massive shaker since 1868.

The U.S. Geological Survey has identified five kinds of soil types in the Bay Area that affect the shaking a building might receive in the event of an earthquake.

If you're fortunate, your home is perched on bedrock, which does not contribute greatly to shaking amplification. On the other end of the spectrum is "Soil Type E" — which includes water-saturated mud and artificial fill. When this soil is exposed to violent shaking, the water and the fill mix together, essentially turning into quicksand.

Photo: Association Of Bay Area Governments

This map measures the expected severity of shaking across the Bay Area in the event of a major earthquake in the northern San Andreas Fault system.

This map measures the expected severity of shaking across the Bay...

Naturally, any structures with shallow foundations sitting atop this geological gruel face the prospect of major damage, tilting and even collapse. Especially at risk are soft first-story buildings — garage or open storefront on the first floor and residence on top — a typical San Francisco style.

"All the areas built on fill [soft mud and sand], such as the Bayfront, Marina District, Financial District, and SoMa [south of Market], will be very vulnerable," said USGS geophysicist Mary Lou Zoback back in 2006.

The USGS says about 25 percent of the nine-county Bay Area region are rated "Very High," "High" and "Moderate" for liquefaction susceptibility with 145 square miles falling in the Very High category. Only modest shaking would be required to cause liquefaction of deposits mapped with Very High susceptibility.

We decided to see which neighborhoods and towns correspond to the Very High risk areas. The accompanying slide show lists 20 of them in no particular order. Note: This is not an all-inclusive list, merely a selection of some of the better-known neighborhoods and towns.

The following Bay Area neighborhoods are partially or largely built on land susceptible to liquefaction.

The following Bay Area neighborhoods are partially or largely built on land susceptible to liquefaction.

Photo: Silentfoto/Getty Images

Photo: Silentfoto/Getty Images

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The following Bay Area neighborhoods are partially or largely built on land susceptible to liquefaction.

The following Bay Area neighborhoods are partially or largely built on land susceptible to liquefaction.

Photo: Silentfoto/Getty Images

Living in a liquefaction zone — Bay Area's riskiest neighborhoods

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The fact that you live in a high-risk liquefaction zone does not necessarily mean your house is in danger of damage should a temblor strike.

Photo: USGS

These soil types correspond to the USGS' liquefaction susceptibility rankings.

These soil types correspond to the USGS' liquefaction...

The USGS map does not claim to be accurate within 50 meters, nor does it distinguish between old, poorly engineered fill and more modern, engineered artificial fill zones. Also, measures that homeowners may have taken to improve their homes' chances of survival, such as bolting the frame to the foundation, are not factored in.

On the other hand, the 1989 Loma Prieta quake proved just how susceptible liquefaction zones like the Marina District are to violent shaking. The epicenter was 60 miles south of the Marina, yet the quake still severely damaged or collapsed 70 buildings in the neighborhood.

Ironically, much of the Marina's sand, dirt and rubble fill was laid down ahead of the 1915 Panama-Pacifica International Exposition, which celebrated San Francisco's rebirth after the 1906 earthquake and fire that devastated the city. (A more detailed map of the Marina's liquefaction zones can be found here.)

USGS' Zoback estimated that an earthquake of the same magnitude as the 1906 temblor would destroy 40 percent of San Francisco's buildings.

Major earthquakes on the Hayward Fault occur on average every 140 years, according to Brocher — so the fault line is overdue for a temblor by eight years. In some cases, the quakes were only about 90 years apart.